The present invention relates to an X-ray computed tomography, and more particularly to the technique which is effectively applied to the positioning of a rotation center of a scanner of a detection system in a cone-beam X-ray computed tomography.
FIG. 6 is a view showing the general construction of a conventional cone-beam X-ray CT. The conventional cone-beam X-ray CT was divided into imaging unit 1 for carrying out the imaging and image processing unit 2 for processing the detected image data. Control unit 3 carries out the whole control for the imaging unit 1 and the image processing unit 2. In the imaging unit 1, an X-ray source 5 and a two-dimensional detector 6 were arranged in such a way as to be opposite to each other through an object. The X-ray source 5 and the two-dimensional detector 6 were both arranged in a scanner as a scanning mechanism which is rotated around an object 7 with a central axis 9 for the rotation as the rotation center.
The scanner 4 was rotated every predetermined angle and the two-dimensional detector 6 carries out the measurement of the intensity of the transmitted X-rays, which were transmitted through the object 7, every predetermined angle, thereby carrying out the imaging of the transmitted X-ray image of the object 7. The transmitted X-ray image which has been imaged by the two-dimensional detector was converted into the digital image data which was in turn outputted to the image processing unit 2. But, in the following description, the angle of rotation of the scanner 4 is referred to as a projection angle a.
In the image processing unit 2, first of all, the pre-processing such as the gamma correction, the distortion correction, the logarithmic transformation and the non-uniformity correction of the two-dimensional detector 6 were carried out in pre-processing means 10. Next, reconstruction means 11, on the basis of all of the transmitted X-ray images (all of the projected images) after completion of the pre-processing reconstructed the three-dimensional reconstructed image which is the three-dimensional X-ray absorption coefficient distribution within the field of view of the object 7. As for this reconstruction arithmetic operation method, there is known the cone-beam reconstruction arithmetic operation method or the like by Feldkamp described in an article of L. A. Feldkamp et al.: PRACTICAL CONE-BEAM ALGORITHM, Journal of Optical Society of America, A.Vol. 1, No. 6, pp. 612 to 619 (1984) (article 1).
Finally, imaging means 12 subjected the three-dimensional reconstructed image to the image processing such as the volume-rendering processing or the maximum-intensity-projection processing of displaying the resultant image in the form of the two-dimensional image on display means 13. At this time, on the basis of the parameters of a viewpoint, a region and the like to be observed which has been inputted through instruction means (not shown) such as a keyboard, a mouse and a track ball, the imaging means 12 executed the image processing.
In the conventional cone-beam X-ray CT, the scanner 4 mounted with the imaging system including the X-ray source 5 and the two-dimensional detector 6 was rotated and the transmitted X-ray image obtained around the object 7 was imaged, and the reconstruction means 11 obtained the three-dimensional X-ray absorption coefficient distribution of the object 7 placed on the stationary coordinate system fixed to the apparatus body. The stationary coordinate system was defined by the imaging system, i.e., the Z-axis as the rotation center 9 of the scanner 4, and the rectangular Cartesian coordinates on the plane on which the rotation orbit of an X-ray focus 14 of the X-ray source 5 lies (hereinafter, referred to as xe2x80x9ca mid-planexe2x80x9d for short, when applicable), i.e., the X-axis and the Y-axis.
The position of an X-ray beam 8 imaged by the detection elements of the two-dimensional detector 6 was specified by an angle (projection angle) a between the straight line which passes through the orbit of the XYZ coordinate system for the X-ray focus 14 to reach the two-dimensional detector 6 and the X-axis, xe2x80x9cthe rotation-axis projectionxe2x80x9d which was obtained by projecting the rotation center 9 on an imaginary plane (projection plane) 15 is placed on the incident plane of the two-dimensional detector 6, and xe2x80x9cthe mid-plane projectionxe2x80x9d which is the straight line drew by the intersection between the mid-plane and the projection plane. That is, the coordinate axes, when reconstruction the three-dimensional X-ray absorption coefficient distribution of the object 7, were the rotation axis projection and the mid-plane projection on the projection plane.
Since for the actual imaging of the transmitted X-ray image, the continuous analog imaging is not carried out, but the discrete digital imaging is carried out, when performing the reconstruction arithmetic operation, the sampling pitch DP on the projection plane was also required. In addition, a distance SOD extending from the X-ray focus 14 to the rotation center 9, and a distance SID extending from the X-ray focus 14 to the rotation-axis projection 17 were both required. In the following description, the relative positional relationship among the X-ray focus 14, the two-dimensional detector 6 and the rotation center 9 will be referred to as xe2x80x9cthe geometry of the imaging systemxe2x80x9d. The geometry of the imaging system is defined by the distance SOD extending from the X-ray focus 14 to the rotation center 9, and the distance SID extending from the X-ray focus 14 to the rotation-axis projection 17, the sampling pitch DP, the rotation-axis projection and the mid-plane projection on the projection plane 15.
It is well known that of the parameters by which the geometry of the imaging system is determined, the higher accuracy is required for the rotation-axis projection, the mid-plane projection and the sampling pitch than for the the distance SOD extending from the X-ray focus 14 to the rotation center 9, and the distance SID extending from the X-ray focus 14 to the 17 rotation-axis projection. For example, when the effective aperture width of the two-dimensional detector 6 is 30 cm, and the resolution thereof is 512xc3x97512 pixels, the accuracy of the rotation-axis projection, the mid-plane projection and the sampling pitch DP required 0.1 pixel, i.e., about 0.05 mm. This reason is that even if the fine error is present in the positions of the rotation-axis projection and the mid-plane projection, and the sampling pitch DP, the reduction of the image quality is provided for the reconstructed image.
It is known that of the positions of the rotation-axis projection and the mid-plane projection, and the sampling pitch DP, in particular, the rotation-axis projection is important, and even if the fine error is present, generates the remarkable artifact in the reconstructed image. On the other hand, it was difficult to image directly the positions of the rotation-axis projection and the mid-plane projection, and the sampling pitch DP. This reason resulted from the fact that the positions of the rotation-axis projection and the mid-plane projection, and the value of the sampling pitch DP depend on the characteristics of the two-dimensional detector 6 and the installation state of the apparatus.
As for a method of imaging the geometry of the imaging system with high accuracy, there was xe2x80x9cthe X-ray Tomographic Imaging Systemxe2x80x9d described in JP-A-9-173330 (article 2) by the same applicant. In the X-ray tomographic imaging system described in the article 2, first of all, an object (phantom) 19 including a support member 20 and a corpuscle-shaped high absorption member 21 shown in FIG. 7 is arranged in the vicinity of a rotation center 9 (in the position which 3 cm to several centimeters a way from the rotation center 9), and the transmitted X-ray image thereof is imaged from the all-round direction. But, in the following description, the dedicated phantom 19, as shown in FIG. 7, which is used in the correction of the geometry of the imaging system will referred to as xe2x80x9cthe geometry estimate phantomxe2x80x9d or xe2x80x9cthe phantomxe2x80x9d for short.
If after completion of the necessary pre-processing such as the distortion correction and the non-uniformity correction, the transmitted X-ray images for all-round direction were added to each other, for example, as shown in FIG. 8, each of the corpuscle-shaped high absorption members 21 on the phantom 19 would draw an elliptical locus 23 on an added image 34. Since the straight line passing through the centers of the elliptical loci 23 becomes the rotation-axis projection 17 depending on the imaging conditions for the geometry estimate phantom 19, the position CP of the rotation-axis projection could be specified. On the other hand, the position MP of the mid-plane projection was obtained from the change in the length of the diameter in the direction of the rotation center (the minor axis of the elliptical locus 23). That is, the lengths of the minor axes of a plurality of imaged elliptical loci 23, and the positions of the rotation center directions thereof were expressed in the form of the graph, and the position where the length of the minor axis becomes zero was estimated, whereby the position MP of the mid-plane projection was obtained.
For the sampling pitch DP, first of all, for example, a metallic plate in which pin holes are bored at regular intervals, i.e., a hole chart or the like is stuck as a thin object having a predetermined length on the light receiving plane of the two-dimensional detector 6, i.e., the projection plane 15 to image one sheet of transmitted X-ray image. After the transmitted X-ray image had been subjected to the necessary pre-processing such as the distortion correction and the non-uniformity correction, with respect to how many pixels the image size or the hole part of the thin object corresponds to, the sampling pitch DP was obtained by comparison with the actual size.
The present inventors, as a result of examining the prior art, found out the following problems associated with the prior art. In the conventional X-ray CT, as described above, the estimation of the geometry of the imaging system required much work of an operator, for example, and much time. In particular, though the high accuracy was required with respect to the position CP of the rotation-axis projection, in the conventional geometry estimate method, the manipulation by an operator was required and hence there was a problem that a burden was imposed on an operator.
In addition, the accuracy which was obtained by the conventional method of estimating the geometry depended largely on a sense of an operator as a human being, and hence there was a problem that the sufficient accuracy could not be obtained by an operator. In addition, since the work of specification or the like of each of the central positions of the elliptical loci 23 was required, the estimation of the geometry of the imaging system took a lot of time and hence there was a problem that the reduced diagnostic efficiency was shown.
As for the method of solving the above-mentioned problems, the geometry estimation described in the article 2 can also be automatically carried out by executing the image recognition processing and the like. However, since in order to carry out the extremely accurate estimation, the complicated image processing needs to be performed, there is a problem that the manufacture cost of the apparatus is increased.
It is an object of the present invention to provide the technique which is capable of obtaining extremely accurately the position of the rotation-axis projection which contributes largely to the promotion of the high quality image of a reconstructed image and also to provide the technique which is capable of estimating parameters used to define the geometry of the imaging system without depending on a sense of an operator, the technique which is capable of estimating automatically parameters used to define the geometry of the imaging system, and the technique which is capable of improving the diagnostic efficiency.
The objects and novel features of the present invention will be apparent by reference to the description of the present specification and the accompanying drawings. Of the inventions disclosed herein, the typical ones are described as follows.
An X-ray CT according to the present invention includes: a scanner mounted with a detection system having an X-ray source for applying the radial X-rays to a object and imaging means arranged so as to be opposite to the X-ray source and adapted to detect a transmitted X-ray image of the X-rays transmitted through the object (phantom); rotation means for rotating the scanner around the object; reconstruction means for reconstructing a reconstructed image of the object from the transmitted X-ray image; initial-value decision means for deciding an initial value of the position of a rotation center (the position of a rotation-axis projection) of the scanner which is projected on the transmitted X-ray image, wherein the position of the rotation center of the scanner is estimated on the basis of the contrast of the three-dimensional X-ray distribution image which is reconstructed by changing the position of the rotation axis projection decided by the initial value decision means; and an X-ray tomographic image or/and an X-ray three-dimensional image of the object is/are generated from the reconstructed image, which is reconstructed by utilizing the estimated position of the rotation center, to be displayed.
In addition, an X-ray imaging method according to the present invention is an X-ray imaging method for obtaining an X-ray CT image, the method including: the step of collecting a transmitted X-ray image of X-rays transmitted through a object by a scanner mounted with detection means having an X-ray source for generating the X-rays applied radially to the object (phantom) and imaging means arranged so as to be opposite to the X-ray source; the step of deciding previously the position of the rotation-axis projection as the position which is obtained by projecting a rotation center of the scanner on a detection plane of a two-dimensional detector constituting imaging means; the step of reconstructing an X-ray absorption coefficient distribution image of the object from the transmitted X-ray image on the basis of the position of the rotation-axis projection; the step of estimating the position of the rotation-axis projection from the X-ray absorption coefficient distribution image thus obtained; the step of reconstructing a three-dimensional X-ray absorption coefficient distribution image of the object from the transmitted X-ray image; the step of generating an X-ray tomographic image or/and the three-dimensional X-ray image of the object from the three-dimensional X-ray absorption coefficient distribution image thus obtained; and the step of displaying the X-ray tomographic image or/and three-dimensional X-ray image thus obtained. For the estimation of the position of the rotation-axis projection, the position of the rotation-axis projection where the contrast of the X-ray absorption coefficient distribution image obtained from the transmitted X-ray image of the object shows a maximum or a local maximum is specified and estimated as the position of the rotation-axis projection on the transmitted X-ray image.
The property that if the reconstruction arithmetic operation is carried out in the state in which the position of the projected rotation center is deviated, since the artifact of arc shape is generated on the resultant reconstructed image, the contrast is reduced is utilized, and the position of the rotation-axis projection where the contrast of the reconstructed image shows a maximum is decided as the proper position of the rotation-axis projection. As a result, the position of the rotation-axis projection as the parameter used to define the geometry of the imaging system can be estimated using the value independent of a sense of an operator and called the contrast of the reconstructed image. Therefore, the position of the rotation-axis projection which contributes greatly the promotion of the high image quality of the reconstructed image can be obtained with high accuracy.
In addition, the contrast of the reconstructed image is decided as the parameter used to define the geometry of the imaging system, whereby it becomes possible to define the function of the contrast of the reconstructed image in which the projected rotation center is treated as the variable. Therefore, it is possible to estimate automatically the position of the projected rotation center as the parameter used to define the geometry of the imaging system. As a result, a time required to estimate the geometry of the imaging system, i.e., a time required to adjust the X-ray CT can be reduced and hence the diagnostic efficiency can be reduced.
The effects offered by the typical ones of the inventions disclosed herein are simply described as follows. (1) The position of the rotation-axis projection which contributes greatly to the promotion of the high image quality of the three dimensional X-ray absorption coefficient distribution image can be obtained with high accuracy. (2) The parameters used to define the geometry of the imaging system can be estimated independently of a sense of an operator. (3) The parameters used to define the geometry of the imaging system can be automatically estimated. (4) The diagnostic efficiency can be enhanced.
The typical construction of the present invention is summarized as follows with reference to FIG. 1. An X-ray CT includes: a scanner which is mounted with an imaging system having an X-ray source for applying the radial X-rays to a object and imaging means arranged so as to be opposite to the X-ray source and adapted to detect a transmitted X-ray image of the X-rays transmitted through the object; rotation means for rotating the scanner around the object; reconstruction means for reconstructing a reconstructed image of the object from the transmitted X-ray image; and decision means for deciding the position of the rotation-axis projection as the position which is obtained by projecting the rotation center of the scanner on a detection plane of a two-dimensional X-ray detector constituting the imaging means, wherein the position of the rotation-axis projection of the scanner is estimated on the basis of the contrast of a reconstructed image which is reconstructed by using the rotation-axis projection decided by the decision means, and an X-ray tomographic image or/and an X-ray three-dimensional image of the object is/are generated from the reconstructed image which is reconstructed in the position of the estimated rotation-axis projection to be displayed. According to the present invention, the position of the rotation-axis projection which contributes greatly to the promotion of the high image quality of the three-dimensional absorption coefficient distribution image can be estimated with high accuracy.